Constant speed centrifugal pumps may be advantageously used to pump into a pressurized pipe such as used to carry water or sewage (also known as a force main). In such environments, the discharge pressure of the pump may vary for a variety of reasons (e.g., discharge conditions and/or the presence of multiple stations discharging into a common force main). Since the pump discharge flow rate is dependent on the total head or pressure that the pump must overcome (generally, the higher the head level, the less the pump will discharge), in environments where the total head in the discharge main that the pump must overcome varies, the discharge volume from the pump will vary, with relatively high flow at low force main pressures and relatively low flow at high force main pressures.
However, some pumping applications require that a consistent flow be discharged notwithstanding the variable discharge pressures, with varying flow potentially leading to issues such as pump damage, overloading of the motor, and/or failure to pump the liquid from the station leading to overflow conditions.
Various pump operations have been used in an attempt to maintain a flow rate in environments where outlet pressure fluctuates. For example, U.S. Pat. No. 7,040,868 addresses air plugs encountered by fire engines (i.e., where the pump is expected to intermittently pump gasses) and uses an engine governor which controls the operating speed of the engine, with the controller overriding the governor to control the engine.
Still other designs have been used for controlling pumps based on various encountered conditions, including those disclosed in U.S. Pat. Nos. 6,178,393, 7,308,906, 8,249,826, 8,417,483, 8,444,394, 8,540,493, 8,801,389 (Publ. No. US2011/0076156), and U.S. Pat. No. 8,840,376 (Publ. No. US2010/0247332).
The present invention is directed toward providing a pump and method in which a constant flow rate will be reliably delivered notwithstanding changing pressure in the system.